This project seeks to characterize forward and reverse sound transmission through the human middle ear. One of the most significant discoveries in hearing during the past twenty years has been that a healthy ear not only detects sound but also generates sounds. This "reverse energy flow" originates in the cochlea, travels through the middle ear and can be measured as otoacoustic emissions in the ear canal. The accessibility of otoacoustic emissions to measurements makes them a potentially useful tool for objective evaluation of cochlear and middle-ear function. However, little is known about the effect of the middle ear on otoacoustic emissions and thus our view of the inner ear is clouded by incomplete knowledge of the middle ear. To understand how the middle ear affects forward and reverse sound transmission, measurements will be made before and after specific modifications to the middle ears of human cadavers. Measurements will include middle-ear pressure gain from the ear canal to the inner ear (forward pressure gain), middle-ear pressure gain from the inner ear to the ear canal (reverse pressure gain), middle-ear input impedance, and ear-canal pressure to stapes-velocity transmission. Examples of middle-ear modifications include fixation of the stapes (as in otosclerosis), fixation of the incus-malleus head, and eardrum perforations. These measurements will be used to develop mathematical models of the human middle ear for normal and pathological conditions. To test the applicability of the mathematical models to living subjects with normal and pathological middle ears, air-hone gap and otoacoustic emissions will be measured in human subjects. Measurements on subjects will be compared with results of the mathematical models. These comparisons are intended to test the applicability of the mathematical models to living humans and to allow us to devise ways to remove the confounding effects of the middle ear from measurements of otoacoustic emissions and thus increase the general utility of otoacoustic emissions as a diagnostic tool for detecting cochlear dysfunction.